The examples discussed below relate generally to medical fluid delivery. More particularly, the examples disclose systems, methods and apparatuses for the control of fluid flow in kidney failure treatment systems.
Due to various causes, a person's renal system can fail. Renal failure produces several physiological derangements. The balance of water, minerals and the excretion of daily metabolic load is no longer possible and toxic end products of nitrogen metabolism (urea, creatinine, uric acid, and others) can accumulate in blood and tissue.
Kidney failure and reduced kidney function have been treated with dialysis. Dialysis removes waste, toxins and excess water from the body that would otherwise have been removed by normal functioning kidneys. Dialysis treatment for replacement of kidney functions is critical to many people because the treatment is life saving.
Hemodialysis and peritoneal dialysis are two types of dialysis therapies used commonly to treat loss of kidney function. A hemodialysis (“HD”) treatment utilizes the patient's blood to remove waste, toxins and excess water from the patient. The patient is connected to a hemodialysis machine and the patient's blood is pumped through the machine. Catheters are inserted into the patient's veins and arteries so that blood can flow to and from the hemodialysis machine. The blood passes through a dialyzer of the machine, which removes waste, toxins and excess water from the blood. The cleaned blood is returned to the patient. A large amount of dialysate, for example about 120 liters, is consumed to dialyze the blood during a single hemodialysis therapy. Hemodialysis treatment lasts several hours and is generally performed in a treatment center about three or four times per week.
Another form of kidney failure treatment involving blood is hemofiltration (“HF”), which is an alternative renal replacement therapy that relies on a convective transport of toxins from the patient's blood. This therapy is accomplished by adding substitution or replacement fluid to the extracorporeal circuit during treatment (typically ten to ninety liters of such fluid). That substitution fluid and the fluid accumulated by the patient in between treatments is ultrafiltered over the course of the HF treatment, providing a convective transport mechanism that is particularly beneficial in removing middle and large molecules.
Hemodiafiltration (“HDF”) is another blood treatment modality that combines convective and diffusive clearances. HDF uses dialysate to flow through a dialyzer, similar to standard hemodialysis, providing diffusive clearance. In addition, substitution solution is provided directly to the extracorporeal circuit, providing convective clearance.
Peritoneal dialysis uses a dialysis solution, also called dialysate, which is infused into a patient's peritoneal cavity via a catheter. The dialysate contacts the peritoneal membrane of the peritoneal cavity. Waste, toxins and excess water pass from the patient's bloodstream, through the peritoneal membrane and into the dialysate due to diffusion and osmosis, i.e., an osmotic gradient occurs across the membrane. The spent dialysate is drained from the patient, removing waste, toxins and excess water from the patient. This cycle is repeated.
There are various types of peritoneal dialysis therapies, including continuous ambulatory peritoneal dialysis (“CAPD”), automated peritoneal dialysis (“APD”), tidal flow APD and continuous flow peritoneal dialysis (“CFPD”). CAPD is a manual dialysis treatment. The patient manually connects an implanted catheter to a drain, allowing spent dialysate fluid to drain from the peritoneal cavity. The patient then connects the catheter to a bag of fresh dialysate, infusing fresh dialysate through the catheter and into the patient. The patient disconnects the catheter from the fresh dialysate bag and allows the dialysate to dwell within the peritoneal cavity, wherein the transfer of waste, toxins and excess water takes place. After a dwell period, the patient repeats the manual dialysis procedure, for example, four times per day, each treatment lasting about an hour. Manual peritoneal dialysis requires a significant amount of time and effort from the patient, leaving ample room for improvement.
Automated peritoneal dialysis (“APD”) is similar to CAPD in that the dialysis treatment includes drain, fill, and dwell cycles. APD machines, however, perform the cycles automatically, typically while the patient sleeps. APD machines free patients from having to manually perform the treatment cycles and from having to transport supplies during the day. APD machines connect fluidly to an implanted catheter, to a source or bag of fresh dialysate and to a fluid drain. APD machines pump fresh dialysate from a dialysate source, through the catheter, into the patient's peritoneal cavity, and allow the dialysate to dwell within the cavity, and allow the transfer of waste, toxins and excess water to take place. The source can be multiple sterile dialysate solution bags.
APD machines pump spent dialysate from the peritoneal cavity, though the catheter, to the drain. As with the manual process, several drain, fill and dwell cycles occur during APD. A “last fill” occurs at the end of CAPD and APD, which remains in the peritoneal cavity of the patient until the next treatment.
Both CAPD and APD are batch type systems that send spent dialysis fluid to a drain. Tidal flow systems are modified batch systems. With tidal flow, instead of removing all of the fluid from the patient over a longer period of time, a portion of the fluid is removed and replaced after smaller increments of time.
Continuous flow, or CFPD, systems clean or regenerate spent dialysate instead of discarding it. The systems pump fluid into and out of the patient, through a loop. Dialysate flows into the peritoneal cavity through one catheter lumen and out another catheter lumen. The fluid exiting the patient passes through a reconstitution device that removes waste from the dialysate, e.g., via a urea removal column that employs urease to enzymatically convert urea into ammonia. The ammonia is then removed from the dialysate by adsorption prior to reintroduction of the dialysate into the peritoneal cavity. Additional sensors are employed to monitor the removal of ammonia. CFPD systems are typically more complicated than batch systems.
In each of the kidney failure treatment systems discussed above, it is important to control ulrtrafiltration, which is the process by which water (with electrolytes) moves across a membrane, such as a dialyzer or peritoneal membrane. For example, ultrafiltratrion in HD is a result of transmembrane and osmotic pressure differences between blood and dialysate across a dialyzer membrane. For a given osmotic pressure, the greater the transmembrane pressure the more rapid the ultrafiltration.
People with kidney failure typically retain water and fluids between treatments. That excess fluid needs to be removed during the next treatment. It is important to know how much fluid is removed so that the patient can be returned to their normal weight by the end of therapy. It is also important in some instances to know accurately the rate at which ultrafiltration is taking place at a given time during therapy.
Different systems have been employed to control ultrafiltration. One system described in U.S. Pat. No. 5,247,434 (“the '534 patent”), assigned to the assignee of the present application, the entire contents of which are incorporated expressly herein by reference, controls ultrafiltration volumetrically. The patent describes a volumetrically balanced system that uses first and second chambers of substantially equal volume. Each chamber includes two compartments, one termed a “pre-dialyzer” compartment and the other a “post-dialyzer” compartment. Each opposing “pre” and “post” compartment of a chamber is separated by a flexible diaphragm. Solenoid-actuated valves control the filling and emptying of each compartment. In general, each compartment is completely filled before its contents are discharged. Also, the “pre” compartments are alternately filled and discharged and the “post” compartments are alternately filled and discharged. Filling a “pre” compartment causes a discharge of a corresponding and opposing “post” compartment, respectively. Filling a “post” compartment causes a discharge of a corresponding and opposing “post” compartment.
Since the volumes of opposing “pre” and “post” compartments of the two chambers are equal, the system volumetrically balances the flow of dialysate to and from the dialyzer. One benefit of this volumetrically controlled system is that dialysate flow to the dialyzer can be accurately measured over a wide range of flow rates.
The volumetric system works well for HD machines placed in centers, which produce dialysate online. In HD, the dialysate is not infused into the patient and is therefore not considered a drug. The balancing chambers can therefore be located inside the machine and sterilized between treatments. The same balancing chambers are used over and over.
PD infuses dialysate into the patient's peritoneum. Dialysate for PD is therefore considered a drug, so that the dialysate has to meet sterility requirements for a drug. Anything that comes in contact with the dialysate must also be sterilized and discarded after use. For PD then, at least a component of the balancing chambers would have to be sterilized and disposable, making balancing chambers for PD less attractive from a cost standpoint, compared for example to simple tubing used with peristaltic pumps.
Problems exist with prior fluid control systems employing scales to measure the weight of fluid delivered to and taken from the patient. For example, previous systems employing scales have had to be robust enough to accommodate the total size and weight of the dialysate used during treatment. The load cells of systems have an associated error, which is based on a percentage of the total weight of fluid. As the total weight of the fluid increases, the error increases correspondingly and begins to compromise the accuracy of the system.
Also, because all the bags have to be weighed, a relatively robust mechanical base hanging system has to be provided to handle the associated stresses. Further, the size of the weighing system makes it more prone to interferences from bumps or hits for example. The size of the weighing system can also make storage of the multiple bags difficult, for example, providing a container or support system capable of isolating the weighing system from mechanical interferences and protecting the load cell.
Moreover, multiple supply bags can be complex and difficult for the patient to attach to the weighing system. For example, the multiple bags may have to be lifted to an inconvenient height. Also, in prior gravimetric systems, spent dialysate has to be collected, requiring the operator to carry heavy, full bags of fluid twice, once for setup and again after treatment. A need therefore exists for a simplified and relatively inexpensive fluid control system for kidney failure treatments, which is accurate and easy to maintain.